An apparatus for mounting a tread on the outer periphery of abase tire is known among tire manufacturing apparatuses. On this apparatus, a separately manufactured base tire, which serves as the base of a tire, and an annular tread, which has been cured in advance, are joined together as they are brought closer to each other by a plurality of units holding them separately.                Specifically, such a tire manufacturing apparatus consists of a drum shaft, a rim assembly, a tread expanding means, a tread pressing unit, and a moving mechanism. The drum shaft, extending horizontally from the machine body, rotates as it is driven by a motor. The rim assembly provided on the drum shaft holds a base tire in such a manner as to allow air to be injected into the base tire. The tread expanding means has a plurality of bar members (fingers) for simultaneous expansion or contraction, the bar members, which have a plurality of rollers protruding and disposed in two upper and lower rows, arranged at equal distances on a circumference. And the moving mechanism moves the tread expanding means and the tread holder toward the rim assembly. On this apparatus, an annular tread is first held on the plurality of bar members such that the inner periphery of the annular tread is radially expanded, and a base tire is secured to the rim assembly. Then the tread radially expanded by the tread expanding means is moved to a predetermined position relative to the base tire, and then the tread is brought closer as the bar members come in contact with the outer periphery of the base tire. Finally the tread is held down to the base tire by the tread pressing unit and then the bar members are pulled out, thus accomplishing the integration of a base tire and a tread.        
Also, another tire manufacturing apparatus is of such a structure that the bar members are advanced and retracted individually. Since the bar members can be freely advanced or retracted relative to the support member supporting the bar members, pulling the bar members out from between the base tire and the tread will accomplish the integration of a base tire and a tread.
Further, its tread expanding unit is provided with a sensor to control the advance and retraction of the bar members. And when the tread is placed on the bar members, both the edge portions of the tread are detected as they pass along the sensor, and the width center of the tread is calculated therefrom. Then the positions at which the tread is held on the respective bar members are detected. Now the bar members are advanced or retracted individually according to these detections so that the edge of the tread comes in contact with a positioning member provided at a predetermined position on each bar member. Thus the tread is aligned with respect to the tread expanding unit.
However, in the former case, the tread radially expanded by the tread expanding means may not often be mounted on the base tire in a correctly aligned condition. Therefore, there may be cases where the tread is defectively mounted on the base tire with waves in the width direction or in an oblique orientation. Also, in the latter case, the tread may be held aligned on the bar members of the tread expanding unit, but the alignment of the tread is accomplished with the edges of the tread brought into contact with the positioning members. When, for example, the tread is a wing type, that is, when both edges of the tread are molded thin-walled, the thin tread edges tend to get turned up or deformed when they are brought into contact with the positioning members. Thus, when the tread is mounted on the base tire, the turned-up or deformed portions, which remain on the tread on the base tire, may affect the tire performance negatively. Furthermore, in both the former and latter cases, the bar members are pulled out with the tread pressing unit holding down the annular tread to the base tire. This may cause a friction between the annular tread and the bar members and thus lead to a defective mounting of the tread on the base tire with waves in the width direction